Numerous devices and methods have been employed in the past for mode-locking of various lasers. Most of those methods have relied on special components and separate alignment procedures to establish the mode-locking.
Active mode-locking uses an amplitude modulator inside the cavity to change the loss at a rate equal to the laser round-trip frequency, resulting in a pulse train. Although it is relatively simple to design and implement, active mode-locking has several limitations. One of the major disadvantages of active mode-locking is that it is hard to scale the pulse width down, which generally ranges from several picoseconds to tens of picoseconds.
Passive mode-locking relies on the intensity-dependent loss element in the laser cavity. When the loss of the cavity is decreased with increasing intensity, pulse formation and shaping follows. This allows the pulse to shape itself after passing through the mode-locking element and thus allows for much shorter optical pulses, as compared with active mode-locking.
Two types of passive mode-locking devices are the fast saturable absorber and the slow saturable absorber. A slow saturable absorber device has a response time that is longer than the pulse duration, while a fast saturable absorber has a response time that is faster than the pulse width. Thus, with the proper balance of dispersion and nonlinearity in the laser cavity, fast saturable absorbers allow for shorter pulse durations than slow saturable absorbers.
A common implementation of a slow saturable absorber is a so-called saturable absorber mirror, which is based on some type of a quantum well structure, where changes in absorption and the time response of the excited carriers allows for the desired loss modulation in the cavity. Such devices operate in both reflection and transmission. Similar techniques have been employed with graphene-based saturable absorbers and other types of saturable absorbers where the material itself provides for intensity-dependent loss dynamics. Typically, such devices require special fabrication steps and frequently require special alignment.
Another class of saturable absorber is based on Kerr nonlinearity. One example of this class is additive pulse mode-locking, which is based on Kerr nonlinearity and has been implemented in free space and in fiber-based systems to achieve stable mode-locking. Additive pulse mode-locking relies on interference between two pulses, which have different intensity-dependent nonlinear optical phases. This phase difference yields an intensity-dependent interference when the pulses recombine, and thus allows for intensity-dependent transmission/reflection.